A child’s dress or apron. Allen (3) says there is reason to think that Gilbert had originally named the opera H.M.S. Semaphore (anything nautical to rhyme with “one cheer more”) but changed it to Pinafore at Sullivan’s suggestion. Naming a fighting ship after such an unprepossessing item of apparel must have appealed to Gilbert’s sense of the ridiculous, particularly as it contrasted with such formidable names as Goliath, Audacious, Minotaur, and Majestic (which were in Nelson’s fleet). Then give three cheers for Sir Arthur’s part of the libretto! An incidental note: Shipley (266) imagines that the term arose from a mother’s admonition, “Don’t spot your dress. Here’s something to pin afore it.”